Glass optical waveguide fibers are remarkably strong as they are drawn, but their strength is greatly affected by physical handling. They must therefore be coated with a protective material before they come in contact with the fiber drawing tractors. For a number of reasons including improved diameter control and cleanliness, the tractors should be as close as possible to the draw furnace. At a fixed distance from the furnace, the fiber becomes hotter as draw rate increases. Thus, to cool a fiber by natural cooling, longer distances are required for higher draw speeds. For example, to cook at 125 .mu.m diameter fiber from 1780.degree. C. to a temperature needed to apply a cellulose acetate lacquer solution with acetone, approximately 80 cm of cooling distance is required for a draw speed of 0.5 m/sec. A distance of 120 cm is required to cool that fiber to 50.degree. C. at a draw speed of 0.75 m/sec., and 800 cm is required if the draw speed is 5 m/sec.
The economical production of large quantities of optical waveguide fibers will undoubtedly require fiber drawing speeds greater than 1 m/sec. If no fiber cooling means is employed and if the distance between the furnace and the coating apparatus is not sufficiently long, there may be insufficient time at such higher drawing speeds for the fibers to cool by natural processes to temperatures which are compatible with application techniques employing presently developed waveguide coating materials. When the temperature of the fiber is too high, the coating may become too thin or discontinuous. If the coating is applied from a 100% solids solution, the hot fiber can cause the coating material to set up in the coating apparatus around the fiber, thus preventing any further coating material from being applied to the fiber.
The optical waveguide fiber coating system disclosed in U.S. Pat. No. 4,208,200 employs means for cooling the hot fiber prior to its entering the coating apparatus. The cooling means comprises an elongated, liquid filled container through which the fiber passes. The bottom of the container is provided with a felt wiping die which seals the container and removes excess liquid from the fiber. In a system employing this type of fiber cooling device the fiber can become so hot that it boils the liquid at high draw rates. This causes turbulence which can move the fiber laterally so that it is displaced from its proper position in the diameter measuring device. Also, the wiping means physically contacts the fiber, a factor which might adversely affect fiber strength.
In copending U.S. patent application Ser. No. 318,374, entitled Fiber Cooling Apparatus, filed on even date herewith and assigned to the assignee of the present application, there is disclosed a cooling apparatus which employs a gas rather than a liquid for cooling the fiber. The apparatus disclosed therein comprises an elongated coolant tube through which the fiber passes. Cool, dry helium is flowed into that end of the tube into which the fiber enters. The coolant tube is surrounded by a chamber containing liquified gas. The helium is cooled by flowing through a coil submersed in the liquified gas prior to being flowed into the tube. Since the flow of helium into the coolant tube is subtantially perpendicular to the fiber axis, the fiber begins to vibrate after a maximum permissable flow rate is exceeded.